This patent application describes a method of controlling common-mode (CM) voltage range in differential transconductor-capacitor (gm-C) circuits. It can also be applied equally as well to more generalized VCO, filter and amplifier circuits.
Differential gm-C circuits offer good design trade-offs for speed and power for high frequency circuitry. Filters using transconductors and capacitors are often called xe2x80x9cgm-Cxe2x80x9d filters. A transconductor is a circuit that has a voltage as an input and a current as an output. Most of the integrated gm-C filters use transconductance tuning vis a PLL, or transconductance fixing with an external precision resistor. However, differential gm-C circuits typically suffer from limited linear range, and the requirement for a common-mode control circuit. This need arises in both state-space and bi-quad structures, because of their dependence on integrator building blocks. The goal for a good integrator design is to maximize output impedance for high DC gain, and to minimize degradation of complex pole xe2x80x98Qxe2x80x99 factors. The high impedance outputs are loaded with differential or grounded capacitive loads. Differential filter structures, such as state-space or ladder structures, include differential feedback to control the differential signal excursions, however there is no implicit control of the common-mode (CM) voltage of amplifier outputs.
This is usually accomplished by sensing the CM level through a pair of differential amplifiers between the output and a CM reference, but large swing non-linearity can cause differential to common-mode conversion and added distortion. Lower distortion circuits can be created using resistors to sense the CM level, but these have the disadvantage of increasing output conductance, reducing xe2x80x98Qxe2x80x99 factors for bi-quad structures.
CM control circuits also must have settling time constants widely separated from the actual filter poles, either much higher or much lower. This is to prevent the differential signal flow and the common-mode signal paths from interacting.
Typical gm-C type integrators and some CM control methods can be found in Y. Tsividis and J. O. Voorman (editors), Integrated Continuous-Time Filters: Principles, Design, and Applications, EEE Press, Copyright 1993.